Process of esterifying hydrated cellulose with lower fatty acids



Patented May 8.

UNITED STATES PATEN OFF-ICE.

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ll'o Drawing.

This invention relatesto processes of esterifying hydrated cellulose with lower fatty lysts. Another object of the invention is to provide a process of making cellulose esters of the lower fatty acids in a plurality: of stages, certain of the acid groups being combined into the ester during the'first stage without use of anhydr ids and catalysts, .and the remainder of the acid groups-being combined in the ester by the subsequent treat ment of the esters formed in the first stage, thus effecting an economy in expensive ingredients, such as the anhydrids of the acids. Another object of the invention is to provide a process ,in which the first-stage esterification is performed in the absence of any ma' terial more hydrolytic than the fatty acids themselves, thus leaving the cellulosic mater'ial essentially unchanged in its physical appearance by the first stage of the process. Other objects will hereinafter appear.

In our application, Serial N 0. 122,028, filed July 12th, 1926, for cellulose esters of fatty acids and processes of making the same we have shown. that unmodified or undegraded cellulose can be esterified, under proper conditions by the lower fatty acids without the use of catalysts or expensive anhydrids and the like. In the case of each of the lower fatty acids, approximately one molecular proportion of acid is combined withan amount of cellulose corresponding to 24 carbon atoms. This first-stage ester is then esterified further by the usual process, but withnotabl'e economies in the use of anhydrids and with inild catalysts to avoid molecular degradation.

We hav now found that cellulose esters of the lower fatty acids may be prepared in which approximately 4 molecular proportions of acid are combined with an amount of cellulose corresponding to 24 carbon atoms and without the use of anhydrids or cata lysts,in fact without'modifying the physi Application filed January 12, 1927. SerialNo. 160,765.

cal appearance of the treated cellulosic material. We are able to bring about this result by using hydrated cellulose as the raw mate rial to be heated'with the acid as the sole acylating agent. These first-stage esters prepared by simple heating of the hydrated cellulose with the acids as the sole acylating agents are very stable, as indicated by their resistance to hydrolysis by prolonged boiling in water. In general the fibers of the new compounds are superficially indistinguishable from the hydrated cellulose forms, such as rayon fibers, from which they are prepared.

It is a useful fact that these esters are more readily esterifiedthan plain cellulose. The practical effect is that when these four in twenty-four esters are further esterifi'ed by using anhydrids, or even acyl chlorids, in the Ways previously described for the acylation of cellulose itself, savings of from 25% to 40% of the amount of anhydrid or chlorid may be obtained.

Our invention is concerned with the esters of the fatty acids having more than 1 and less than 8 carbon atoms, such as acetic, propionic, butyric, valeric, caprylic, heptylic, etc. The rate of esterification falls off rapidly with the higher members of this series of acids. Consequently acetic acid, propionic acid and butyric acid are those Which are preferred, because of their relatively greater commercial availability and their greater speed of reaction. Acetic acid is commercially the most important at'present, because of its lower cost. 7

\Ve preferably use hydrated cellulose which has not been so degraded by hydrolysis or 4 oxidation as'to yield esters that form brittle films. We have found, for example, that reverted cellulose, in the form of viscose rayon fibers or cuprammonium rayon fibers,

form, such as flakes, grains or powder is also convenient. J

The first-stage esterifi'cation'may be conducted over a considerable temperature cellulose and the products produced from it.

Within this preferred range the temperatures vary with the boiling points of the different acids. For example, those acids which boil below 170 C. can be used at atmospheric pressure, a considerable excess of acid over the amount which will enter the ester being used. But-We find that the time can be very considerably shortened by using a superatmospheric pressure, say in an autoclave, provided the corresponding temperature does not reach the point where the product is degraded. While we use the acids as the sole acylating agents and prefer to employ them liquids, nevertheless, the. esterification can be effected, when the acids are mixed with non acylating liquids, such as chloroform, carbon tetrachloride, benzene, toluene, xylene, chloro-benzene, etc. But these produce no marked advantage, other than facilitating removal of water.

During the. first-stage acylation it is desirable to kee very low. T e total water present, includ ing the water in the original ingredients, plus the water formed during the esterification, should never be allowed to reach the point where it prevents the introduction of more than 6% of the acyl group. We aim to maintain the conditions during the reaction such that the water is removed or di ininished in quantity. The use of a fractionating column is one way of obtaining this result, the water vapor being allowed to.

Pam dut while the fatty acids are condensed 'and flow back tothe bath. This is possible because all of the fatty acids having more than 1 and less than 8 carbon atoms boil considerably above the boiling point of water. The use of a reflux condenser, while useful with all of the acids, can be dispensed at atmosp It is especially advisable wit with more readily with the lower members of the series, such as acetic and pro ionic the higher members of the series. r

r the first stage of esterification the hydrated cellulose is heated in the acid-until the four in twenty-four ester is formed. For example, we may boil 1 part of hydrated cellulose in 30 parts by weight of acetic acid the operation being carried out hericapressure. The acid is initially substanti 1y free from water, say of .99 to 100% strength. The boiling oint of the'reaction bath is thus kept at 11 0. or slightly above. Of course, any suitable preca ltlons' are taken to pggvent the loss of acids its vapors ing carried away, the same being condensed and returned to without admixture with otherthe amount of water present the bath by the use of any of the expedients familiar to chemists.

As the boiling in acetic acid is continued under these conditions the esterification progressively takes place and the acetyl groups in the ester reach the point where their proportion is not materially increased by further boiling. Roughly the proportion of acetyl is 21% of the weight of ester. This full esterification may be obtained after different lengths of time with different sam ples of cellulose. proximately this percentage after 200 hours of treatment. But a use tained after a much shorter time. For instance, the percentage of acetyl in the ester reaches 13% before 96 hours in practically every instance. This 13% ester has the property of being readily esterified further in the usual baths but with economy in acetic anhydrid. It shouldbe noted that in the above discussion, as well as in the claims,

Some of them reach ap 111 product is obwe have referred to the percentage of the For instance, 14% of the acetyl group means about 19.5% of combined acetic acid.

One typical example of the acetylation of a reverted cellulose, such as cuprammonium silk, is given in the following table:

Per cent The first column gives the hours of treating of the sample with acetic acid boiling at atmospheric pressure. The second column gives the corresponding percentages of acetyl group in the product.

After this first-stage etherification, the excess ofacid is removedif the ester is to be further esterified in a bath containing an anhydridand a catalyst. Part of the acid can, however, be left with the four in twenty-four ester to function as a diluent or a solvent during the second-stage acyla- 'tion, as is usual in the ordinary acetylation of plain cellulose. If the acetylated fibers from the first stage, are desired to be use in that condition, they are, of course, thoroughly washed in water and dried, in which event they look like the hydrated fibers from which they were derived. Instead of the 30 parts of acetic acid in the above example,

of acetyl group.

. go above 170 The excess of acid vagpr we may employ 30 parts by wei ht of propionic or butyric acid and employ, preferably, a reflux condenser throug which the water vapors may pass, while retaining the acid vapors. The time of boiling is convenientl about 200 hours.

When 30 parts of heptylicacid are used with 1 part by Weight of hydrated cellulose, the temperature should be kept below 1659 C. and preferably above 145 C. This is under the boiling point of the acid, but sufficient to obtain a useful esterification without too serious degradation.

Whenit is desired to work at superatmospheric pressures, 1 part of hydrated cellulose may be heated, for example with 30 parts of acetic acid by weight of 99 to 100% strenh at a temperature between 140 and 160 say in a vessel of the autoclave type. The time of esterification is enormously shortened under these conditions, being generally less than hours.

The esterification may even be carried out with the vapors of the. acids. For example, the hydrated material may be placed in an acid-resisting tube heated by any suitable means, such as an oil bath, and the vapor of acetic acid conducted through the tube. This 0 eration may be conducted either at atmosp eric ressure or subatmos heric pressure, provided the temperatures 0 not may be repassed or recirculated, prefera after slightly reheating, through the tube. This enables a rapid acetylatlon to take lace. The subsequent esterification of the rst-stage ester can take place in any of the usual esterifying baths, but with the amount of anhydrid diminished by from to 40%.

, Moreover, the increased susceptibility of our four in twenty-four esters to further esterification' enables us to carry out the latteroperation without the use of strong catalysts, thus avoiding the danger of degradation when such catalysts as. sulfuric acid are employed. We can. operate with a milder and safer one, such as zinc chlorid or magnesium perchlorate trihydrate. As such esterification methods have been previousl described, it is unnecessary to go over t em in detail here.

Having thus described our invention,

what we claim as new and desire to secure by Letters-Patent is:

the acet l I 1. In the process of making a cellulose ester of a fatty acid, heating together hydrated cellulose and a fatty acid having more than 1 and less than 8 carbon atoms without a catalyst at a temperature between 100 and 170 C. inclusive until substantially 4 molecular proportions of said fatty acid combine with an amount of cellulose atoms, in'the absence of any anhydrid and.

catalyst, the action being stopped while the ester, thus produced, still has substantially the same appearance as the original hydrated cellulose, and thereafter esterlfyin it further with the aid of an anhydrid o a fatty acid and a catalyst.

. 4. In the process of making'a fatt -.acid

cellulose ester, heating hydrated ce lulose between 100 and 170 C. with a fatty acid having more than 1 and less than 8 carbon atoms without any catalyst 'until substantially 4 molecular proportions of said fatty acid combine with an amount of cellulose corresponding to 24 carbon atoms, said acid being the sole acvlating agent, and thereafter further esterifying it with the aid of an anhydrid of a fatty acid and'a catalyst.

5. In the process of making cellulose acetate heating hydrated cellulose in an excess of acetic acid at a temperature between 100 and 170 C. in the absence of'a catalyst, the acid being the sole acetylating agent, until cup in the .ester reaches at least 6%, an t ereafter further acetylating the ester thus produced in the resence of acetic anlfilydrid and a milder cata yst than sulfuric aci Signed at Rochester, New York, this 7th day of January, 1927.

HANS r. CLARKE. CARL J. MALM. 

